A configuration of a motorcycle in which a rear swing arm is provided to a rear part of a motorcycle body is disclosed, e.g., in Japanese Patent Application Laid-Open Publication No. 2002-87364 (JP 2002-87364 A). The relationship between the rear swing arm and a brake caliper as disclosed in the above document will be described with reference to FIGS. 11A through 11C hereof.
As shown in FIG. 11A, a rear swing arm 103 is attached to a motorcycle body frame 101 via a pivot shaft 102. A rear wheel 104 is rotatably attached to a rear end of the rear swing arm 103. A brake disc 105 is integrally attached to the rear wheel 104. A caliper 106 is provided to a rear portion of the rear swing arm 103. The caliper 106 clamps the brake disc 105 and performs a braking function.
When the rear wheel 104 is not subjected to an upward force from a road surface, the rear swing arm 103 will extend downward at a slant from the pivot shaft 102. An angle βa is formed by a line Vc, which passes through a center of a rear wheel axle 107 and the caliper 106, and a line VL, which is plumb with the road surface.
As shown in FIG. 11B, when upward force from the road surface acts on the rear wheel 104, the rear wheel 104 will be raised a distance Sb from the initial position, and the rear swing arm 103 will extend from the pivot shaft 102 so as to be substantially parallel to the road surface. An angle βb formed by the line Vc and the line VL is greater than the angle βa.
As shown in FIG. 11C, when considerable upward force from the road surface acts on the rear wheel 104, the rear wheel 104 will be raised a distance Sc from the initial position, and the rear swing arm 103 will extend upward at a slant from the pivot shaft 102. An angle βc formed by the line Vc and the line VL is greater than the angle βb.
During braking, the rear swing arm 103 is subjected to braking force from the caliper 106. When the total of the braking force and the force received from the road surface acts on the rear swing arm 103, torsion and flexing occur in the rear swing arm 103 according to the total force.
The changing of the orientation of the caliper 106 relative to the road surface as indicated by the angles βa, βb, βc causes deformation to occur in the rear swing arm 103. This deformation may vary slightly from the predicted deformation.
As a result, the contact between the caliper 106 and the brake disc 105 (degree of contact) varies, and an abnormal noise known as “brake squeal” may occur. A need exists for a technique that can resolve these defects.